It is known practice to separate air in a double column consisting of a medium-pressure column and a low-pressure column, the top of the medium-pressure column being thermally connected to the bottom of the low-pressure column by means of a reboiler-condenser. The medium-pressure column may for example operate at a pressure of between 5 and 6 bar abs and the low-pressure column between 1.2 and 1.5 bar abs. If the low-pressure column operates at a pressure higher than 2 bar abs, then it is commonly said that the apparatus is operating “under pressure” or “at high pressure”. Distillation processes in which the apparatus operates under pressure, whether to produce impure oxygen (typically 95 mol%) or pure oxygen (typically at least 99.5 mol%) allow an approximately 20% energy saving over a conventional low-pressure design, provided that all of the nitrogen produced is put to use:
The use of pressurized designs therefore imposes two requirements:                all the nitrogen must be used        some of the nitrogen will have to be impure.        
The production of gaseous oxygen (GOX) and gaseous nitrogen (GAN) using two air separation units (ASUs) operating at the same low pressure (LP), and therefore with the low-pressure column operating at less than 2 bar, can be set out as follows:
ProductionTrain 1: LP ASUTrain 2: LP ASUTotalGOX202040GAN10 withdrawn from the MP10 withdrawn from the100columnMP column40 withdrawn from the LP40 withdrawn from thecolumnLP columnTotal GAN: 50Total GAN: 50Energy100100200
In the context of a production unit having several air separation units in parallel (referred to as a “multi-train” unit), the invention proposes combining the use of a pressurized design for at least one train and a low-pressure design for at least one train.
The nitrogen needed can be produced first of all by the pressurized set, then by the low-pressure set if the requirement is higher than the maximum production of the pressurized set.
The proportion of impure nitrogen derived from the pressurized set will either be purified in the low-pressure set or purified and produced in the pressurized set using a fluid derived from the low-pressure set.
Regeneration of the pressurized set can be achieved using a residual gas from a low-pressure set.
This arrangement makes it possible to produce the desired quantity of nitrogen at the desired purity (preferably pure) and enjoy, for the production of oxygen, some of the energy saving by using low-pressure designs for some of the trains. EP-A-2489968 describes a process according to the prior art.